In recent years more and more modern wireless standards are based on Orthogonal Frequency Division Multiplexing (OFDM) modulation. In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. The orthogonality allows for high spectral efficiency, robustness against narrow-band co-channel interference and robustness against intersymbol interference (ISI) and fading caused by multipath propagation.
Example standards based on OFDM modulation include Wireless LAN (WLAN) 802.11a/g/n/ac/ah, 3GPP Long Term Evolution (LTE), Data Over Cable Service Interface Specification (DOCSIS), Digital Video Broadcasting (DVB), Ultra-Wideband (UWB), mobile WiMAX, Power Line Carrier (PLC), etc.
Several disadvantages of OFDM modulation include sensitivity to Doppler shift which limits its use in high speed vehicles, sensitivity to frequency synchronization problems and high peak-to-average-power ratio (PAPR). The high PAPR requirement is a major problem in transceiver design that requires the use of very linear transmitter circuitry, especially so for the RF power amplifier circuitry.
One simple solution to operating the power amplifier circuit in a linear mode with large PAPR is to transmit less power and thus avoid compression. This technique is known as Back-Off (BOF). In this case, however, there is a very dramatic drop in efficiency. FIGS. 7A and 8 illustrate the relationship between linearity, efficiency and output power. Furthermore, output power becomes very limited such as in the case where an output average power requirement of 20 dBm dictates saturation power levels of 34 to 38 dBm. Various other prior art power amplifiers provide different solutions for amplifying OFDM signals but all of them are sub-optimal.
There is thus a growing need to provide an optimal power amplifier that is (1) wideband, (2) highly linear and (3) highly efficient in its operation.